1. Field of the Invention
This invention relaates to a copying machine which is capable of forming copies on both sides of the copying sheet by a simple operation.
2. Description of the Prior Art
It has been the general practice in the art to perform copying operations only on one side of each copying sheet. However, when large quantities of copied sheets are to be kept in storage, the volume to be stored becomes twice that of books or other writings in which both sides of each leaf are printed, and hence there arises the problem of storage space, weight and the waste of material. The need for a double-sided copying machine (whereby copying can be made on both sides of each copying sheet) is also apparent when one considers copying costs.
Several double-side copying systems are now known. Among them is a system in which two sensitive drums are provided, each being designed to perform copying on one of the two sides of each copying sheet, and images are formed separately on both sensitive drums and transferred respectively to both sides of the transfer paper. This system is simple in principle, but as it requires two sets of copying apparatus, the machine in bulky and very expensive. There is also known a system in which the image for one side is first transferred from the sensitive drum to an intermediary drum and temporarily retained thereon while the image for the other side is again formed on said sensitive drum, and both images are then transferred from said drums to opposite sides of the same transfer paper. Although this system requires only one set of image forming apparatus, it involves the technical problem of performing the transfer operation twice through an intermediary drum. According to still another system known in the art, a normal copying operation (transfer and fixing) is first performed on one side of the transfer paper, and then this transfer paper is guided back to the original (starting) position or into a second paper feeder and hence again fed into the copying apparatus with the transfer paper being turned over so that the same copying operation may be practiced on the back side of the transfer paper. This system necessitates only one set of copying apparatus; the same as employed for one-side copying and hence the copying machine used therefor can be reduced in size and cost. It may therefore be said that this system is the most practical and economical.
This third system, however, still involves the difficult problems of (1) smoothly reversing the transfer paper, after completion of copying on one side, when it is again fed back into the copying apparatus (2) turning up the edges of the transfer papers, after completion of copying on one side, when they are stacked up in the paper feeder, (3) delivering the transfer papers one by one, without overlapping each other, from the paper feeder when they are again fed into the copying apparatus, (4) adapting the system to a voluminous two-side copying operation, and (5) adapting the system to a two-sided copying process in which either one-side or two-side copying can be selectively performed as needed. These outstanding problems have been a barrier to the practical application of this last system.
In order to increase efficiency and speed-up the copying work, there has also been devised a so-called cassette system in which a plurality of copying sheets can be set in a cassette. Generally, according to this cassette system, various types of copying sheets differring in size and quality are stacked in the respective cassettes, and when practicing the copying operation, the desired cassette is selected and loaded into the paper feeder unit of the copying machine. This system allows easy exchange of the copying sheets by the simple changing of cassettes. As an improvement, there has also been devised a so-called double-deck system according to which two types of cassettes, which differ in size with respect to each other, are beforehand set in the same copying machine and either of these cassettes can be easily selected by simply pushing a selection button to carry out copying on the desired sheets.
However, in actual commercial use, copying sheets of a certain size (such as A size or B size) are used predominantly, and there are few occasions where other sizes of sheets are used.
Further, according to the conventional cassette system, when using cassettes of different sizes, it becomes necessary to remove the cassette which has already been set in position and then to insert a new cassette, and in some cases, it is also necessary to replace the new cassette with the original cassette after completion of the copying operation.